1. Field of the Invention
The present invention relates to a video signal reproducing apparatus, such as an LD (Laser Disk Player) or a VTR (Video Tape Recorder), and more particularly to a video signal reproducing apparatus which uses a video memory and can reproduce images having a little deterioration in a variable speed reproduction mode.
2. Description of the Prior Art
In a variable speed reproduction mode, such as a slow or quick mode, of a video signal reproducing apparatus, such as an LD, an image signal is reproduced in such a manner that track jump is performed so as to output an identical frame twice, or the reading skips over an intermediate frame. However, the above variable speed reproduction has a disadvantage in that a periodical abnormality of voice reproduction occurs because identical voice signals are successively output twice or an intermediate part of voice signal is skipped in the same manner as the image signal.
A method using a field memory is known that is intended to eliminate the above disadvantage. This method uses a video memory having a capacity of at least one field of video signal. A read clock applied to the video memory is fixed at a frequency. A reference synchronizing signal with respect to a reproduced video signal from the LD or the like (for example, a reference synchronizing signal for spindle servo) is made variable. The reproducing speed of the reproduced video signal (for example, the revolution velocity of the laser disk) is varied by changing the frequency of the reference synchronizing signal. According to the above method, when the reproducing speed of the LD is changed, video signals are written into the field memory in synchronism with a write clock dependent on a change in the reproducing speed, while the written video signals are read therefrom in synchronism with the read clock having the fixed frequency. In this manner, the video signal and the audio signal can be correctly reproduced.
There is a phenomenon, called "passing", in the variable speed reproduction. This is due to the fact that the frequency of the write clock and the frequency of the read clock differ from each other. When the frequency of the write clock is higher than that of the read clock, the time relation between the write clock and the read clock is reversed, and the write timing passes (outruns) the reading timing (see time relation of V.RESET signals in FIG. 6). On the other hand, when the frequency of the write clock is lower than that of the read clock, the read timing passes the write timing (see time relation of V.RESET signal in FIG. 8). When the passing takes place, upper and lower portions of the field image becomes different from each other because of the passing. In practice, as the contents of successive field images are similar to each other, a particular countermeasure is not employed.
A frame synchronizer can also handle the passing phenomenon as mentioned above. The frame synchronizer is a device that synchronizes two video signals with each other. Normally, the reproduced video signals in various video reproduction apparatuses may have different frequencies within a standard. One of the video signals is written into the frame memory in synchronism with the frequency thereof, and is read therefrom in synchronism with the frequency synchronized with the other video signal.
An application device of the frame synchronizer uses a field memory. However, the following problems arise from passing in such an application device. Normally, the frame synchronizer with the field memory only stores image data in the field memory, and a synchronizing signal is added to the image data read out from the field memory. When passing has occurred, writing of one field image data is prevented or image data of the same field is successively read twice, in correspondence with the difference between the memory write speed (frequency) and the memory read speed. Hence, an image of the same field (the first field or the second field) is successively read from the memory twice. In this case, when the synchronizing signal is added to the output signal from the memory, a field reversal occurs. That is, the images of the first field are scanned at the scanning position of the second field and the images of the second field are scanned at the scanning position of the first field, as shown in FIG. 1. With the above in mind, in the frame synchronizer, the reading of a field image from the memory after passing is delayed by one line, and the synchronizing signal is added to the delayed field image. With this operation, the field image after the passing is scanned at the scanning position one line below the respective fields as shown in FIG. 2, it is possible to prevent reversal of the relation between the scanning positions of the first and second fields.
The general frame synchronizers are applied to cases where the difference between the frequencies of the two signals is approximately within .+-.1 percent. Therefore, passing occurs at a long period and the swinging of displayed images resulting from the above-mentioned scanning position control is not so considerable.
However, in devices having a variable speed reproduction function such as a Laser Karaoke, considerable swinging of display images frequently takes place because the difference between the frequencies of the memory write clock and the memory read clock is approximately .+-.10percent.